| Industrial crystallization process is usually happened with the crystals suspended, the fluid dynamics characteristic as fluid flow state and the suspension state of crystals and the crystallization kinetics factors jointly determine the quality of the crystal products. Research proves that in the same liquid field of continuous mold, different sizes of particles have different distribution, so the CSD is different at different locations of the mould, and the product crystal size primarily depends on the discharge point CSD. So study on the CSD of different position of continuous crystallization process crystallizer can provide a reference for controlling the crystallization process and mold design.Most of the early crystallization research adopted MSMPR model, and without considering fluid dynamics factors inside the crystallizer, lead to large difference with practical industrial crystallization process.We must consider the both influence of fluid dynamics and crystallization dynamics so as to reduce the difference. But because of the particle’s nonlinear flow, uneven distribution and different fluid flow states, no accurate crystal suspension density distribution can be obtained by experiment. CFD simulation technology is widely adopted in crystallization process, it can be used as a powerful tool for studying on the liquid flow field and particle state of suspension of the crystallization. Our group established the multiphase flow and multi-scale coupling model of crystallization according to the multi-scale distribution features of crystal inside the crystallizer. The model is verified and applied in this paper.The Oslo crystallizer adoped in This article is widely used in the crystallization process, the research conclusions are as follows:1. Simulated the fluid dynamics state in the Oslo crystallizer with the help of ANSYS 14.5.Studied the CSD condition in different liquid fields of different circulation rates; and studied the CSD of different setted zones, on account of that analyzing the relationship between the product CSD and the Oslo mould’s flow state.Through the study of CSD in different areas of the crystallizer, it has confirmed that stable CSD can be realized in the crystallization area at suitable circulation fluid import velocity. Required crystal size is directly related to the circulation rate.At the same time, variable diameter area can realize crystal size grading, controlling crystal size and crystal suspension density in the circulate liquid. The research conclusion can provide theoretical and engineering design reference for the crystallization process.2. Optimizing the setted multi-scale coupling model, then applied it to the Oslo crystallizer. Simulated the flow field by CFD software, using the related information as the simulation results of the micro scale. Divide the Oslo crystallizer reasonably based on the particle suspension density distribution. Modify the solute material balance and the population balance equation of the setted mesoscopic scale model correspondingly to deduce the Oslo crystallizer mesoscopic scale model of this simulate condition.Then we got the multiphase flow and multi-scale coupling model applied in Oslo crystallizer crystallization process.3. Simulate the continuous cooling crystallization process of KCl by using the multiphase flow and multi-scale coupling model of Oslo crystallizer crystallization process,and verifid the model.Studied the distribution information of related flow fields, crystal suspension state and supersaturation at different circulation inlet velocity and different feeding crystal volume fraction.With the fixed mould structure and other conditions,the simulation result shows that:the circulation velocity has large influence on the flow field distribution, and then affect the crystal distribution and the crystallization process;circulating rate has certain influence on the homogeneity coefficient of solution supersaturation distribution, along with the rising of the circulation rate, solution supersaturation distribution Tend to be more uniform;With the regional location in the mould rises,in each area of crystallizer, the average crystal suspension density and the degree of the solution supersaturation decrease no matter which circulation rate;the overall distribution situation of average crystal suspension density and supersaturation degree are same in most areas. As the feed crystal volume fraction increases, the number of the crystals in the crystallizer increase;Feed crystal volume fraction has certain degree influence on distribution uniformity of saturation.On the basis of the result of the model simulated, studied the distribution rule of crystals and supersaturation degree of each region in different circulation rates and different feeding crystal’s volume fractions, accounting for the rules by crystal growth theory, the obtained results conform to the basic theory of fluid dynamics and crystallization process, verified the correctness of the multiphase flow and multi-scale coupling model of crystallization processes,which can be used to guide the crystallizer design and crystallization process analysis. |
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